This invention relates to nuclear fuel enclosed in metal cans and aims to improve the integrity of the closure of such cans.
Before a can closure is made during manufacture of canned fuel the can interior is evacuated (and may be filled with an inert gas) to remove any foreign substance which may set up, subsequently, chemical reactions which deleterious effect the integrity of the can as a container of nuclear fuel. In spite of these precautions, which are a routine step in canned nuclear fuel manufacture, it is difficult to ensure that the removal of foreign substances has been absolute because some gases or vapours, especially water vapour remains adsorbed in solid surfaces within the can. The inventor has detected that certain failures of canned fuel can be ascribed to the chemical reactions which occur within the can between the gaseous components of these sorbed vapours, said components appearing in the free state only after the can has been subject to irradiation and high temperature in a nuclear reactor. Making good the defects at this stage is a rather drastic procedure involving reactor shut down and replacement of part of its core to extracate the faulty fuel can.
Amongst metal can materials, zirconium is an excellent base for an alloy from which to make the can and its closure, on account of its low neutron absorption cross section and combined with other alloy constituents, its good high temperature performance. Zirconium base alloys, such as those sold under the trade name "Zircalloy", are prone to absorb hydrogen, and as a consequence react with that gas to form zirconium hydride.
There will almost certainly be some remnant water vapour inside a closed can, the vapour acting as a potential, subsequent source of free hydrogen. It has been found that the attack by this free hydrogen on the can closure, accompanied by the formation of the hydride, which migrates to the seal weld of the closure, results in a breach of the can integrity.